Polyethylene (PE), and in particular high density polyethylene (HDPE), is the most commonly used material for the production of pipes. Polyethylene used for the manufacture of HDPE pipes needs to meet certain mechanical criteria, such as impact resistance, toughness and scratch resistance, as well as chemical requirements, e.g. resistance to corrosion. The pipes are often used at high inner pressures and subjected to external mechanical forces. Although the overall pressure is usually well below the yield stress of the polymer, mechanical failure almost always occurs before the polymer is chemically degraded. It is generally accepted that this is due to the existence of local heterogeneities of micrometer size in the polyethylene pipe causing a strong localized stress distribution around the flaws that exceeds the yield stress. Such a stress concentration induces the formation and growth of a craze by rupture of the craze fibrils. In this respect it is of high importance to use PE with as low local heterogeneities as possible. Normally these heterogeneities originate from supported catalysts where, especially when metallocene catalysts are concerned, silica or other related inorganic carriers are used.
Polyethylene pipes are particularly suited for non-conventional pipe installation due to their flexibility, deformability and availability in long lengths. The widespread use of modern relining techniques and fast pipe installation practices call for high material requirements and guarantees of performance, particularly with respect to the effect of scratches, notches, nicks and impingements that are inherent to these techniques and facilitates slow crack growth (SCG). When installing pipes by modern no-dig or trenchless installation methods (e.g. pipe bursting, horizontal direction drilling) the pipe is dragged horizontally through the ground. While often highly advantageous in that the surface of the ground, e.g. roads and other installations, need not be disturbed and the installation cost significantly reduced, on the other hand, the no-dig methods give the disadvantage of a high tendency for protruding stones, rocks etc. to scratch the outer surface of the pipe in the longitudinal direction. Furthermore, at the bottom of such longitudinal scratches, there will be a very high local tangential stress when pressure is applied inside the pipe. Thus, unfortunately, such scratches are very harmful since they often start cracks propagating through the wall that would otherwise never even have started.
These requirements on the performance level of pipes, in turn, mean that the polyethylene used for their production must meet certain requirements. Generally polyethylene used for pipe production has the following properties:
PropertyUnitsSuitable rangeMolecular weight (Mw)g/mol100,000-500,000MFR5g/10 min0.2-1.4 (EN12201)Densityg/cm3935-960
Commerically available polyethylene for pipe production is generally prepared either by using a chromium or a Ziegler Natta catalyst. Monomodal HDPE made in a single reactor with a chromium (Phillips) catalyst gives a relatively poor property profile with respect to demanding pressure pipe applications. HDPE pipe made using Ziegler Natta catalysts are usually prepared with two reactors operating in series; one reactor making a lower molecular weight homopolymer and one reactor making a higher molecular weight polymer containing comonomer which gives a better property profile compared to monomodal chromium HDPE. Ziegler Natta catalysts enable high molecular weight, high density polyethylene to be produced which provides the polyethylene with its required mechanical properties. The disadvantage of the use of Ziegler Natta catalysts, however, is that the polyethylene tends to have inhomogeneous comonomer incorporation.
Metallocene catalysts are attractive to use in polyethylene pipe production because they achieve much more homogeneous comonomer incorporation in the polymer compared to Ziegler Natta and chromium catalysts. Here, homogeneous comonomer incorporation means that comonomer is incorporated in similar quantities into polymer chains across the whole molecular weight range. In contrast with Ziegler Natta catalysts comonomer is typically incorporated only in polymer chains with certain molecular weight. The improved comonomer incorporation property with metallocenes will improve significantly, for example, slow crack growth and rapid crack propagation behaviour of the polymer which has crucial impact on the pipe properties.
Currently metallocene catalysts are exploited to a much lesser extent commercially for the production of polyethylene for pipe production than Ziegler Natta catalysts. When metallocene catalysts are employed in commercial scale processes, they tend to be used on external carriers or supports. The use of supports avoids the problems of reactor fouling, poor polymer morphology and low polymer bulk density which are typically encountered with the use of unsupported metallocenes. Supported metallocene catalysts, however, have relatively low activities and invariably yield polyethylene of relatively low molecular weight which means they are not suitable for pipe production. Due to the low polymerisation activity, supported metallocene catalysts also yield polyethylene with high ash content and high gel content. As described above, due to local heterogeneities in the polymer structure high ash content and high gel content, often lead to mechanical failures in the pipe, meaning cracks and breakages. They also often affect the pipe appearance and performance by introducing roughness on the inner and outer surface which has an effect e.g. on the flowability of liquids. Also, high ash content has an effect on the electrical properties of the polymer leading to higher conductivity.
Silica is typically used as a carrier in supported metallocene catalysts and thus remains in the produced polymer. Silica is a hard material and will scratch steel. Silica particles present in a polymer will scratch the metal surfaces of polymer melt handling equipment, e.g. extruders and dies, both in the polymer production plant as well as in the later melt forming into useful products as the polymer flows along the metal surfaces, under a melt pressure of hundreds of bars. The continued scratching over time results in the polymer melt handling equipment eventually getting worn out.
Also, the level of foreign, e.g. silica, particles in the produced polymer is extremely important because the amount of, e.g. catalyst, residues inside the polymer plays an important role in determining the application where the polymer can be used. For example, film with high strength and clarity, electronics, optical media and pharmaceutical packaging require minimum level of residues in the polymer.
WO98/58001 discloses a process for the preparation of polyethylene for pipe production wherein a multistage polymerisation using a metallocene catalyst is carried out. Hydrogen is present in the first stage of the polymerisation but is entirely consumed therein so that the second stage polymerisation occurs in the absence of hydrogen. The first stage polymerisation produces a lower molecular weight polymer and the second stage polymerisation a higher molecular weight polymer.
WO98/58001 is focussed on the use of supported metallocene catalysts. It teaches that it is particularly desirable that the metallocene complex is supported on a solid substrate for use in the polymerisations. The preferred substrates are porous particulates such as inorganic oxides, e.g silica, alumina, silica-alumina, zirconia, inorganic halides or porous polymer particles. All of the examples in WO98/58001 employ supported metallocene catalysts.
WO98/58001 teaches that its process yields a polyethylene having a MFR2 of 0.01 to 100 g/10 min, a weight average molecular weight of 30,000 to 500,000 g/mol, a melting point of 100-165° C. and a crystallinity of 20 to 70%. The examples of WO98/58001 illustrate the preparation of numerous polyethylenes. The MFR2 values of the polymers produced is always greater than 1 g/10 min (c.f. the above 0.01 g/10 min minimum of the range) and in many cases is significantly greater with some examples producing polymers having MFR2 values of 43 and 32 g/10 min. None of the polyethylenes produced in the examples of WO98/58001 have a MFR2 of <0.1 g/10 min (MFR5=0.2-0.5 g/10 min for pressure pipe) which is the ideal value for polyethylene pipe production. As shown in the examples section later, this is consistent with the Applicant's finding that it is not possible to produce polyethylene suitable for pipe production (i.e. high molecular weight and low MFR2) using the supported catalyst illustrated in WO98/58001.
US2011/0091674 discloses multimodal copolymers of ethylene and their preparation in a multistage polymerisation process carried out in the presence of a metallocene catalyst. The catalyst is used in solid form, either on a particulate support such as silica, on solidified alumoxane, or as solid particles prepared using emulsion solidification technology.
WO2013/113797 discloses a process for the production of multimodal polyethylene using a three stage polymerisation process. WO2013/113797 is focussed on the use of a Ziegler Natta catalyst system for the polymerisation process.
WO2013/091837 discloses bridged bis(indenyl) ligands, methods for their preparation, and their use in the preparation of metallocene complexes which may be used in the polymerisation of ethylene.
There is a need to develop a metallocene based polyethylene polymerisation process which proceeds with low reactor fouling and high activity and which yields a polyethylene suitable for pipe production. The polyethylene must have a high molecular weight, a low MFR5, a high bulk density (indicating good particle morphology) and ideally a low ash and gel content.